Soldering of printed circuit boards which are incorporated into electric home appliances such as televisions and videos is carried out by flow soldering since such boards must be mass-produced inexpensively. Flow soldering can solder the entire surface of a printed circuit board in a single operation. Therefore, it is superior to other types of soldering with respect to mass productivity. An automatic soldering machine for carrying out soldering by flow soldering includes processing units such as a fluxer, a preheater, a wave soldering bath, and a cooler, and an endless conveyor runs above these processing units. When printed circuit boards are soldered in an automatic soldering machine, they successively undergo application of flux by the fluxer, preheating by the preheater, adhesion of solder in the wave soldering bath, and cooling by the cooler to perform soldering while they are transported by the conveyor. Each processing unit installed in an automatic soldering machine can be used for years under ordinary conditions of use, but a wave soldering bath has a shorter service life than the other processing units. The reason therefor is the occurrence of erosion of a wave soldering bath.
This erosion of a wave soldering bath is a phenomenon in which a constituent part of the wave soldering bath is locally worn away by the action of the molten solder contained therein. If erosion develops in a wave soldering bath, a hole forms in the body of the bath, causing molten solder at a high temperature to spill out. Molten solder which spills out of a wave soldering bath creates an extremely dangerous situation in which not only is the wiring of the automatic soldering machine or the floor of the workplace scorched, but in which workers carrying out soldering operations are burned. Therefore, some countermeasure is taken so that erosion does not take place in a wave soldering bath. The most effective countermeasure is to use stainless steel for the constituent parts of the wave soldering bath. Stainless steel has a strong oxide film of chromium or nickel formed on its surface, and this surface prevent direct contact of the metallic portion of stainless steel with the molten solder contained therein. As a result, with a stainless steel bath body, it is difficult for the molten solder to alloy with the stainless steel, and erosion of the bath body is decreased to that extent.
However, erosion sometimes takes place even with a stainless steel bath body. Thus, when the strong oxide film formed on the surface of stainless steel disappears and the metallic portion of clean stainless steel is exposed, erosion of stainless steel takes place. Namely, if the oxide film on the surface of stainless steel locally disappears for some reason, the Fe in the stainless steel and the Sn in the molten solder make an alloy. Since the resulting FeSn alloy has a decreased melting point, the alloy melts into the molten solder. This phenomenon spreads to the periphery and the interior, and eventually a hole forms in the stainless steel.
The cause of the local disappearance of an oxide film from the surface of the stainless steel of a wave soldering bath is that molten solder energetically flows in a wave soldering bath. The energetically flowing molten solder rubs the stainless steel bath body, whereby the oxide film on the surface of the stainless steel are physically peeled off. Therefore, erosion of a wave soldering bath develops frequently in a portion where there is energetic flow of molten solder. The portion where erosion occurs severely in a wave soldering bath is the periphery of a pump installed in the bath where the flow of molten solder is rapid and particularly in a portion of the bottom surface of the bath body which is situated beneath the pump installed in a duct.
A conventional wave soldering bath will be explained while referring to a FIG. 3. FIG. 3 is a front cross-sectional view of a conventional wave soldering bath. The body of the wave soldering bath 1 has the shape of a lidless box entirely made of stainless steel. Molten solder 3 is contained in the bath body 2 and is heated to a molten state and maintained at a predetermined temperature by an unillustrated electric heater. A first discharge nozzle 4 and a second discharge nozzle (not shown) are installed in the bath body 2. A large number of discharge holes 5 are bored in the first discharge nozzle 4.
The first discharge nozzle 4 is connected to a duct 6, and a flow straightening plate 8 having a large number of holes 7 bored therein is installed in the upper portion of the duct. An impeller pump 10 having a large number of blades 9 radially mounted thereon is installed in an end portion of the duct 6. A shaft 11 is secured to the upper portion of the impeller pump 10, an unillustrated sprocket is mounted on the upper end of the shaft, and the sprocket is driven by an unillustrated motor. An inlet 12 is formed in the duct 6 it its bottom portion where the impeller pump 10 is installed. The inlet 12 is slightly smaller in diameter than the diameter of the blades 9 radially mounted on the impeller pump 10.
The state of flow of molten solder in the above-described conventional wave soldering bath will be explained below. First, when the unillustrated motor is driven, the unillustrated sprocket is rotated, and the shaft 11 secured to the sprocket is rotated, thereby rotating the impeller 10 to which the shaft 11 is secured. As a result, the molten solder between the large number of blades 9 of the impeller pump is swept by the energy of rotation of the blades 9 and is transported horizontally inside the duct 6. Then the molten solder which is transported horizontally inside the duct 6 has its flow direction changed from horizontal to upwards and moves upwards, and as a result of this change of flow direction, the molten solder becomes turbulent. The molten solder which has become turbulent passes through the large number of holes 7 in the flow straightening plate 8 and undergoes flow straightening. The molten solder which underwent flow straightening by the flow straightening plate 8 is spouted from the large number of discharge holes 5 in the first discharge nozzle 4. The molten solder which is discharged from the large number of discharge holes 5 has a large number of surface irregularities or waves. A printed circuit board contacts the irregularly shaped molten solder and is soldered thereby. The irregularly shaped molten solder readily penetrates into through holes and comers of electronic parts and thus serves to eliminate unsoldered portions. However, the irregularly shaped flow forms bridges between adjoining portions being soldered, and it also forms icicles on the tips of leads. These are corrected by the gentle flow which is spouted from the unillustrated second discharge nozzle.
The situation in which a hole is formed in the bottom surface of the bath body of a conventional wave soldering bath having the above-described structure will be explained. When the impeller pump 10 rotates, the molten solder between the large number of blades 9 of the impeller pump is swept by the energy of the rotating blades and is sent into the duct 6. As a result, the impeller pump 10 sucks molten solder 3 below the duct through the inlet 12 into the space between the blades 9. Because the impeller pump 10 is rotating at this time, the rotation causes molten solder existing below the inlet 12 to rotate and produces a vortex T. Thus, molten solder forms a vortex T above the bottom surface 13 of the bath body 2 in a portion positioned below the inlet 12, and this vortex T rubs the bottom surface 13 of the bath body 2. As a result, the oxide film covering the surface of the stainless steel on the bottom surface 13 is removed so that the naked metal is exposed in that portion, and Fe in the stainless steel makes an alloy with Sn in the solder. As this alloying proceeds, it becomes erosion K and eventually a hole ends up forming in is the bottom surface 13.
The cause of erosion in a wave soldering bath is also related to the Sn content in the solder and the soldering temperature, i.e., the temperature of molten solder in the wave soldering bath. Namely, the higher the Sn content in solder used in a wave soldering bath, the more easily erosion occurs. This is because, as stated above, Fe in stainless steel alloys with Sn in the solder and thereby causes erosion, and the higher the content of Sn in solder, the more alloying with Fe progresses. In addition, the higher the temperature of molten solder in a wave soldering bath, the more easily erosion develops. This is because a higher temperature not only promotes alloying of Fe and Sn but the resulting alloy more easily melts into high temperature molten solder.
In the past, solder used in soldering of printed circuit boards was Pb-63Sn alloy solder. This solder has an Sn content of approximately 60%, and the temperature of solder in the wave soldering bath is 220-240° C. With this Sn content and solder temperature, it was difficult for erosion of stainless steel to take place. However, because conventional Pb—Sn solder contains Pb, its use has come to be regulated. Namely, in view of an adverse effect of Pb when it accumulates in the human body, “lead-free solder” containing no Pb has come to be used in recent years. Lead-free solder contains Sn as the main component (at least 95 mass percent), to which Ag, Cu, In, Bi, Zn, Ni, Cr, Mo, Ga, Ge, P, or the like is suitably added. Since lead-free solder containing Sn as a main component often has a melting point of at least 220° C., the soldering temperature, i.e., the temperature of molten solder in a wave soldering bath must also necessarily be made high, and in general it is 250-260° C. Thus, with lead-free solder, the Sn content is higher and the temperature of solder in a wave soldering bath is also higher compared to the past. As a result, erosion takes place more frequently in a wave soldering bath using lead-free solder.